In a case of variable compression-ratio mechanism adapted to change a compression ratio of the engine in accordance with a rotational position of a control shaft, a large load such as combustion load and inertia load is repeatedly applied to an actuator for driving the control shaft. Hence, Patent literature 1 discloses a previously proposed technique. In this technique, a speed reducer is interposed between the actuator and the control shaft, and thereby a torque by which the actuator holds the control shaft is lightened so that consumption energy of the actuator is reduced when the engine compression ratio is maintained constant.